As we come on the air on this anniversary of 911 new worry about al qaeda. The terrorist organation is increasing recruits from an unlikely source, americans. More than 100 signed on including inside syria. Brian ross is here now to tell us who they are. Brian? Reporter: Diane, u.S. Law enforcement and intelligence officials tell abc news that as many as 100 people in this country are under surveillance tonight in this country as potential terrorists and it's a kind of american brigade. They come from small towns and big cities law enforcement officials tell abc news, including, they say, a man who lived here on monte vista road in phoenix. 30-year-old aerk harroun was in the u.S. Army on the day of the 9/11 attacks. This year he became one of about a dozen americans who authorities say are fighting in syria with a group who has sworn allegiance to al qaeda. You're going down in flames. Reporter: His father says his son fell in with the wrong people. He's the furthest thing from the terrorist there could be. Reporter: But lured back to the u.S. By the fbi he will soon stand trial on charges he helped al qaeda. That's the most disturbing thing is to see americans switching sides and going over to the enemy. Reporter: Recruits to al qaeda are showing up in hot spots across the middle east and afghanistan. At least 50 young american men, many from minnesota have been tracked to the al qaeda group fighting in somalia. This is the real disneyland. Come here and join us. Reporter: It's a place where a young man from the small town of gaphne alabama became a top commander. One of the things we seek for is to die as a martyr. Reporter: He's now one of five americans with huge rewards on their head because of their alleged al qaeda leadership positions. Along with men from wisconsin, brooklyn, new york, buffalo, new york and adam gadahn of orange county, california. Gadahn who once tore up his passport on camer is in leadership in pakistan producing videos that urge americans to buy guns and kill their own. What are you waiting for? Reporter: He's the first AMERICAN SINCE THE 1950s TO BE Charged with treason. For obvious security reasons the details of those people who are under surveillance in this country, the 100 or so, are closely guarded. Officials say they're cases in which there is not enough evidence to bring criminal charges yet but more than enough concern to keep a very close eye on. Again some of these u.S. Citizens fighting with al qaeda in syria. That's exactly right. Thank you so much, brian.

This transcript has been automatically generated and may not be 100% accurate.

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